This invention generally relates to dynamoelectric machines, for example, electric motors, and in particular, is directed toward an improved thermostat strap which is inserted between the windings of the motor during motor construction. While the invention is described in particular detail with respect to a single phase induction motor, those skilled in the art will recognize the wider applicability of the invention disclosed hereinafter.
Many single phase induction motors utilize some kind of motor over temperature protection device which functions to open the motor winding circuit whenever an over temperature condition in the winding is sensed. The over temperature protection device generally is electrically connected in series with the source of electrical energy for operating the motor, and is positioned physically between the motor windings so that heat transfer occurs between the windings and the protection device. Consequently, both the current drawn through the device and the surrounding winding temperature are used as inputs to enable the device to exhibit relatively fast response time following an over temperature condition.
Single phase induction motors commonly include a stator assembly constructed from a plurality of individual laminations formed from magnetic material. The laminations include a central opening having a plurality of radially extending slots which communicate with the central opening. When aligned in a stator core, the central opening of the lamination plurality defines a central, rotor receiving bore and the slots define winding receiving receptacles. It is conventional to distribute the windings in the slots according to some predetermined distribution factor. In a 2 pole induction motor, for example, the windings would include a main winding part and an auxiliary winding part, each of which has a first pole and a second pole physically offset in relation to one another. Each pole is constructed from a plurality of coils of wire, each coil of wire being constructed from a plurality of individual wire turns. The wire coils include a generally straight conductor run portion of the wire turn which is carried in the slots of the stator assembly, and an end turn portion of the wire turn which extends outwardly of the end laminations of the stator core, between respective ones of that wire turn part forming the straight conductor run portions of the coil.
The end turn portions of the motor windings long have been a problem area in motor construction. The end turns are not an important factor in flux generation but account for a relatively large part of the resistance loss exhibited by the motor winding. Ideally, the end turns are made as small as possible. The end turns often are subjected to various blocking processes designed to compact the wire mass of the end turns into some predetermined silhouette so that the overall motor dimensions meet a particular application's dimensional requirements.
Since the motor over temperature device generally is placed between the windings after all other motor constructional steps are completed, some provisions must be provided to permit the later insertion of the protection device.
One form of motor protector finding wide application is available from Texas Instruments Incorporated under the trademark KLIXON. In particular, the device is a bimetallic sensor designed to separate or cause an open circuit in the motor winding when an over temperature condition is sensed in that winding. The actual protector operative elements are enclosed in an irregular shaped, metal outer casing. In the past, it has been conventional to use some type of insulative material, available from the E. I. DuPont de Nemours & Company under the trademarks MYLAR and NOMEX, for example, to form a thermostat strap for later reception of the protecting device. In general, these materials are relatively flexible. They are, however, good insulators, an important factor in view of the fact that the windings and outer casing both are metallic materials.
Use of MYLAR and NOMEX thermostat straps, however, has been plagued with difficulties. In the first place, the material itself tends to be relatively expensive, especially when the thermostat strap must be formed individually, which generally is the case. Because prior strap devices were constructed from flexible material, they often compressed during end turn blocking, making the later insertion of the protector difficult. Even when compression of the thermostat strap does not occur, blocking or other manufacturing processes often cause the position of the prior art thermostat straps to shift. It is difficult, if not impossible, to reposition the strap in its proper position after the motor construction is completed. For example, it is common to immerse or coat the stator assembly and motor winding in a suitable epoxy varnish. After cure, the strap cannot be moved without destroying the stator assembly. Position of the thermostat strap is important in the operation of the protecting device. The thermostat, protector or protecting device, terms used synonymously in this specification, operation depends in part on the fact that the protector is in close proximity to a large number of the end turns of the coils forming the winding. That proximity permits the protector to follow any temperature change in the winding closely, and to open the motor circuit if the temperature change becomes excessive.
Motors utilizing motor protectors sometimes are mounted in applications or transported in an inverted attitude so that means for counteracting gravitational or other forces must be provided in order to prevent the protector from falling from the thermostat strap. Thermostat straps having such means are known in the art, but they have been difficult to manufacture and their operation has been time consuming in that each protector must be locked in place individually following its insertion in the thermostat strap.
Our invention eliminates these prior art deficiencies by providing a molded thermostat strap having a generally semi-rigid construction. The thermostat strap includes a central well having releasable locking means associated with it, which automatically engages the protector upon insertion in the well, locking the motor protector in position. The strap also includes means insertable in the slots of the stator core for preventing the shifting of the strap during blocking or other motor construction steps. Because the device is semi-rigid, it does not crush during end turn blocking. The strap includes a central well having strap wings extending outwardly from it. The wings preferably are formed with a predetermined radius of curvature so that the strap tends to conform to the end wire configuration. The ability to preform the strap wings reduces the possibility of the thermostat strap being forced from the winding during blocking. Strap construction is facilitated in that the strap may be manufactured utilizing mass production plastic molding techniques.
One of the objects of this invention is to provide a low cost thermostat strap for use with dynamoelectric machines.
Another object of this invention is to provide a thermostat strap having means for releasably securing a motor protector to the strap.
Yet another object of this invention is to provide a thermostat strap including means for preventing shifting of the strap.
Yet another object of this invention is to provide a thermostat strap having a semi-rigid construction.
Still another object of this invention is to provide a thermostat strap having a central well with wings extending outwardly from and on opposite sides of the well, the wings having a predetermined radius of curvature imparted to them.
Other objects of this invention will be apparent to those skilled in the art in light of the following description and accompanying drawings.